Geology Reference
In-Depth Information
prominently in geomorphic processes. Particularly impor-
tant are river and stream patterns because they provide
clues as to the structure of the underlying rocks and
characteristics of the topography (
Fig. 3.34
).
Groundwater accumulates in fractures and the pore
space of rocks and, rarely, as bodies of water in larger
cavities. Groundwater can move laterally and emerge as
springs where it intersects the surface. Erosion produced
by groundwater dissolving certain rocks (typically lime-
stones, rock salt, or gypsum) leads to caverns and a terrain
termed karst topography. Depending upon the stage of
evolution, karst topography may display only a few sink-
holes (collapse pits), numerous sinkholes plus solution
valleys (a collapsed drainage network), or highly eroded
karst in which only pinnacles and spires remain as ero-
sional remnants.
Landforms with the imprint of former lakes, swamps,
and oceans are highly diverse. Typically, these are sites of
sedimentary deposition and, with the removal of water,
leave
flat, broad plains, typi
ed by playas (dried lake
beds). Shoreline processes may lead to features such as
terraces (both erosional and depositional, which may
re
ect former shorelines), sea cliffs, and beaches. Except
for some craters and canyons on Mars, which may have
contained ponded water in the past, and the methane lakes
on Titan, only Earth appears to display landforms associ-
ated with large bodies of water.
3.5.4 Aeolian processes
Aeolian (wind) processes involve the interaction of the
atmosphere with the surface. Most deserts, coastal areas,
glacial plains, and many semi-arid regions on Earth expe-
rience aeolian processes. An atmosphere in motion (wind)
possesses energy, and, as the wind moves over a surface,
some of that energy is transferred to the surface. If we
were to measure the wind velocity at different heights
above the surface, we would see that velocity decreases
toward the surface, as a re
ection of the surface friction.
As shown in
Fig. 3.35
, the changing velocity pro
le
de
nes the boundary layer, within which the air
flow is
turbulent. When plotted on a logarithmic scale, the boun-
dary layer is a straight line, the slope of which is related to
a parameter called the friction velocity. Although this
term is commonly used to describe aeolian processes, it
Figure 3.35. A diagram plotting the wind velocity as a function of
height above a planetary surface; the increase in velocity with height
de
nes the boundary layer, within which the air movement is
turbulent; the velocity decreases toward the surface as energy is
transferred by friction along the surface.
Figure 3.34. Diagrams showing basic stream patterns and relations
to the eroded rocks (from Howard,
1967
, reprinted by permission of
the AAPG, whose permission is required for further use).
